Design of a Concrete Terraced Structure. Vipsanius Incorporated Designs to stand the test of time

Transcription

1 Design of a Concrete Terraced Structure Vipsanius Incorporated Designs to stand the test of time

2 Meet the Team Project Manager: Josh Prines Structural Team: Roberto Cintron, Kwaku Boampong, Nisarg Thakkar, Hal Hamilton Jr.

3 Lower Hill Redevelopment Proposal Architectural rendering courtesy of Bjarke Ingles Group (BIG) 28-acre masterplan; 1,200 residential units, 1 million SF of retail and commerce space An attempt to combine a green network of walking and bicycle paths with the urban street grid of downtown Pittsburgh Promote a sense of sustainability and community; reversing a trend of vacating property in the area

4 Project Overview

5 Site Overview

6 Project Scope Complete design of all above-grade structural members. The following items will be cast-in-place on site: Beams Columns Stairwell Shear Walls Elevator Shear Walls Floor Slabs Construction Management Complete Construction Schedule Above-grade construction estimate Return on Investment Analysis Geotechnical will be subcontracted

7 Advantages of Using Concrete Fire Resistance Affordability Material Availability Durability

8 Floor Layouts

9 Structural Framing Layouts

10 Dead Loads Risk Category: II (ASCE 7-10, Table 1.5-1) Slab: 7.5 thickness Lightweight Concrete (115 pcf) Cast-in-place

11 Dead Loads (Cont.) ASCE 7-10, Chapter C-3 Interior non-load bearing walls 6 thickness Exterior load bearing wall 8 thickness 87 psf Mechanical Allowance 5 psf Elevator Enclosure Wall 8 thickness -100 psf Minimum required 3 for fire resistance (2 hours)

12 Dead Loads

13 Live loads Base values (ASCE 7-10, Table 4-1) Ordinary flat, pitched, and curved roofs : 20 lb/ft² Private rooms and corridors serving them : 40 lb/ft² Roofs used for roof gardens (patios): 100 lb/ft² Determined for each column and girder Reduced according to sections and of ASCE 7-10

14 Tributary Areas

15 Main Wind Force Resisting System Consists of the stairwell and elevator shafts Moment frames were considered but they re not the MWFRS Stairwell needed 5 thickness for fire protection Non-seismic region so shear walls were sufficient

16 Wind loads ASCE 7-10 Directional Procedure (Chapter 27) Complex building geometry Conservative methodology to assign appropriate wind pressures Biggest Challenge: Torsion

17 Wind Load Cases (ASCE 7-10) Four different load cases Only considering the cases with torsion (Case 2 and Case 4) Case 2: Uses 75% of the design wind pressure acting on the principal axis with the torsional moment as shown.

18 Wind Load Cases (ASCE 7-10) Case 4: 56% of the design wind pressure acting on both faces of the building simultaneously. Underestimated eccentricity from this load case LATERAL LOAD ANALYSIS FOR BUILDINGS WITH SETBACK By Victor W.-T. Cheung and W. K. Tso, M. ASCE

19 Wind Design Used tributary areas for each floor to determine a resultant force per floor Used the lever arm to determine the torsion on the MWFRS Torsion results El. (ft) Torsion(kip-ft) Total

20 Wind Load Design We also used an article LATERAL LOAD ANALYSIS FOR BUILDINGS WITH SETBACK By Victor W.-T. Cheung and W. K. Tso, M. ASCE Used to properly calculate the eccentricities for both faces of the building Also to apply shear per floor to analyze the shear walls

21 Snow Load Overview Acts vertically on the structure Supported by the columns Each floor experiences a portion of the load due to stepped patios Wind forces cause a drift on stepped side of building

22 Snow Load Analysis (ASCE 7-10; Chapter 7)

23 Seismic Load Analysis (ASCE 7-10; Chapters 11 & 12) Seismic Load creates a gravity load and lateral load on structure Torsion & Deflection Seismic Design Category A Determine base shear of structure using the equivalent lateral force analysis Load generated by the inertia of the mass of the structure Redistributed to each level as a point load at the center of mass of the structure

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25 Broad Design Two-way and One-way slab (only corridor) Don t need East-West horizontal members in corridor area Beam & Girder will be in same plane They will share tributary area Both will transfer load to column Lateral force resisting system Elevator shaft and shear walls

26 Slab Design Lightweight Concrete (115 pcf) Designed for Deflection (Service limit state) Check for Shear and Flexure Setting Upper Limits to Span- Depth Ratio Simple Useful where spans, loads, load distributions, and member sizes and proportions fall in usual range Calculate Deflection Complex Compared to predicted value imposed by codes Approximate

27 Slab Design One-Way Two-Way Short Direction Long Direction Flexural Reinforcement in both direction Flexural Reinforcement to resist entire load Minimum steel for temperature and shrinkage effect

28 Slab Design Effect of Edge beam on slab thickness α-factor (Beam to slab stiffness ratio) Slab thickness is uniform throughout irrespective of loading. For Fire and Heat 5 One-way portion Two-way portion Minimum thickness required (International Building Code) Selected thickness Sufficient for seismic

29 One-Way Slab Critical Section Minimum thickness- l/28 = 4.3 Correction factor when using light weight concrete : w >= 1.09 Minimum thickness 4.6

30 One-Way Slab Chosen thickness- 7.5 Load Combination: 1.2 w(d) w(l) = psf Maximum moment based on moment coefficients and clear span M u A s reinforcement As is so small so Min A s governs Check for shear and crack control

31 Calculation for One-Way

32 One-Way Slab Cross Section # 4 bar at spacing of 8 (spacing governed by crack control) (short direction) # 4 bar at 15 o.c. (min reinforcement for temp. & shrinkage) (transverse direction)

33 Two-Way Slab Critical Section Biggest span 25 *25 Minimum thickness (ACI): ln/33 (interior) (without beams) ln/30 (corner, exterior) (without beams) When beams are present thickness reduces by 15% Effect of edge beam (αf-factor) (Beam to slab stiffness ratio) Light weight concrete correction factor 7.5

34 Two-Way Slab Chosen thickness- 7.5 Direct-Design Method for Moment distribution Statical Moment (Mo)

35 Two-Way Slab Calculation East-west Slab strip 5 A5 B5 C5 l ln l A_inf reduced live load (ksf) Mo end mid end mid end coeff neg & pos moment (kip-ft) Sum of column moments (kip-ft)

36 Two-Way Slab Cross Section Moment Distribution to slab and beam M u A s reinforcement Column Strip 5 #5 bars Middle Strip 10 #5 bars

37 Horizontal members: Design Microsoft Excel spreadsheet Base assumptions 4-k/in 2 concrete 60-k/in 2 steel Equal reinforcement at top and bottom of beam Includes formulas from ACI manual Also includes references in ACI manual for easy corroboration Optimized with Solver Then manually rounded up to standard bar sizes Some integrated integer constraints are included

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39 Spreadsheet Workings IF(input,output_if_true,output_if_false), OR(input_1,input_2,...) Strength reduction factor φ=if(ε c >0.005,0.9,IF(ε c <0.002,0.65,0.65+(ε c )*250/3)) MAX(input_1,input_2,...), MIN(input_1,input_2,...) Used in combination Maximum transverse-reinforcement spacing s max =MIN(d/2,24,p h /8,12)*IF(V S >4* (f c ')*b*d,0.5,1) Solver Number minimized: Total materials, weighted by tensile strength Adding 1 in 3 of 60-k/in 2 steel = adding 15 in 3 of 4-k/in 2 concrete First, use Ignore integer constraints checkbox to reach optimal solution without encountering errors Integers: Base, height, 2 transverse-reinforcement spacing Then, reënable integer constraints to ensure constructability Manually round bar sizes up to next standard size

40 Horizontal member cross-sections Uniform Section Unique Sections

41 Horizontal member design

42 Column Design Columns are vertical structural members designed to support axial compressive loads, with or without moments Design to support axial compressive load, with moments Supported Loads: Dead & Live Load from floors above Live Roof Load Snow Load Critical load determined using ASCE 7-10, LRFD load combinations Columns are extended into foundation

43 Column Design Alternatives Tied Columns Rectangular Typically used in low seismic regions Economical Spiral Columns Circular Improved ductility and strength Expensive

44 Column Design Procedure (Tied Columns) Determine critical load on column, P u Select trial size and trial reinforcement ratio based on material properties Properties: f c = 4 ksi f y = 60 ksi Determine steel ratio using interaction diagram (use critical load and moment for diagram) Select reinforcement using new steel ratio Check the max load capacity, ΦP n > P u Select ties (ACI Code Section )

45 Column Design Summary

46 Shear Walls (Overview) One shear wall resisting in lateral force wall each stairwell Height of wall above ground = 72 Length of wall = 20 Wall extends into foundation Designed to resist shear & flexure, axial load, and moment/torsional effects Steel reinforcement in front face and back face In both vertical and horizontal directions

47 Shear Wall (Design Procedure) Determine shear force on each floor Select trial rebar and reinforcement ratio based on material properties Minimum percentage of reinforcement in ACI Code Section Check factored moment strength Maximum moment is at the base Accounts for factored axial load (ACI Code Eq. 9-6) Check factored shear strength Maximum shear strength is at the base

48 Shear Wall Design Summary

49 Shear Wall Reinforcement (Cross Section)

50 Elevator Shear Wall ACI Reinforced Concrete Mechanics and Design 6 th edition Results Vertical Reinforcement 2 rows of #8 18 O.C. & 2 #8 4 O.C for support at the corners Horizontal Reinforcement 2 rows of #5 18 O.C

51 Market Analysis and Net Operating Income

52 *Number shown above is the Hard Cost only Pittsburgh (2012) = $ *Both Soft Costs and Land Costs are excluded Pittsburgh (2016) = $184.86

53 *Rental Statistics from 2014*

54 Expected Budget for Construction

55 High-Rent Luxury Apartments in Pittsburgh, PA Aria Cultural District Lofts (7 th Street Across from North Shore) $2.2/sqft (1BR/1BA) sqft The Encore on 7 th Apartments (7 th Street Across from North Shore) $2.3/sqft (1BR/1BR) sqft Flats on Fifth (5 th Ave Crawford Roberts Hill) $2.5/sqft (1BR/1BA) sqft $2.3/sqft (2BR/2BA) 1054 sqft Source: Apartments.com

56 Construction Budget

57 Quantity Take-off & Estimate

58 Quantity Take-off Quantities only include above-grade quantities Subgrade work subcontracted Superstructure Concrete = 5048 cy Structural Framing, Exterior Walls, Shear Walls Reinforcing = 126 tons; 161,500 LF #3, #4, #5, #8, #9, #11 bars C.S.I. Divisions 1-16 (Excluding 2-Site Work) Source: Design Cost Data Archives

59 Concrete Labor Costs Source: Construction Estimating 3 rd Edition (David Pratt)

60 Construction Estimate Lower Hill Redevelopment Project 48,000 SF 1. Fees, Permits, Field Supervision, Insurance, Temporary Utilities 2. Pilings, Earthwork, Utilities 3. Formwork, Cast-in-place, Reinforcement Miscellaneous 6. Rough Carpentry 7. Insulation, Damp proofing, Caulking 8. Doors, Frames, Hardware, Aluminum Windows & Doors 9. Metal Studs, Plaster, Drywall, Carpet, Painting, Staining 10. Signage, Fire extinguishers, Mailboxes 11. Appliances 14. Elevators (1) 15. Basic Materials, Fire Protection, Plumbing, HVAC 16. Basic Materials, Wire Conduit, Panel Boards, Fixtures, Fire Alarms (Labor Included in Values) Return on Investment: 7.4%

61 Construction Schedule Start Date: 2/18/16 Project Completion: 3/17/ Work Days; Approximately 13 months

62 Questions?